1. Field of the Invention
The present invention relates to a method of outputting an image signal, device for outputting an image signal, rangefinder, and imaging device and, more particularly, to a method of outputting an image signal, device for outputting an image signal, rangefinder, and imaging device used in an autofocus camera or the like.
2. Description of the Related Art
Conventionally, a so-called passive rangefinder making use of an image signal output device having a plurality of arrays of light-receiving portions is available as disclosed, for example, in Japanese Patent Publication No. 67203/1991. The principle of range finding of this related art is briefly described by referring to FIG. 7. An image of a subject or an object under measurement 101 is focused onto sensor arrays 103a and 103b via light-receiving lenses 102a and 102b. Reference positions 104a and 104b (hereinafter referred to as “the pair of reference positions”) are set on the sensor arrays 103a and 103b, respectively. This pair of reference positions corresponds to a position where the image is focused if the subject 101 is at infinity. The range to the subject 101 is measured using the principle of triangulation. In particular, where the range (i.e., the distance from the light-receiving lenses 102a, 102b to the subject 101) to be found is D, if images of the subject on the sensor arrays 103a and 103b are focused at a position spaced from the pair of reference positions by a distance equal to n (=n1+n2) pixels, the following equation holds:
                    D        =                              (                          L              ×              f                        )                                (                          p              ×              n                        )                                              (        1        )            where L is the baseline length of the light-receiving lenses, f is the focal distance of the optical lenses, and p is the pitch of the pixels of the light-receiving elements. Since the baseline length L of the light-receiving lenses, the focal distance f of the optical lenses, and the pitch p of the pixels of the light-receiving elements are constants, the distance D can be determined by detecting the number n of the pixels corresponding to the deviation of the subject images on the sensor arrays from the pair of reference positions. A general method of detecting this amount of deviation consists of quantizing the outputs from the sensor arrays 103a, 103b by a computing circuit 105, performing a correlation calculation about the data obtained by the quantization, and finding the amount of deviation from the results of the correlation.
In this passive rangefinder, the difference in sensitivity between the pair of sensor arrays adversely affects the calculation of the correlation, thus deteriorating the rangefinding accuracy. A technique for correcting the sensitivity difference between the pair of sensor arrays is disclosed, for example, in Japanese Laid Open Patent Publication No. 146572/2000. This disclosed technique consists of storing data for correction of sensitivity corresponding to the sensitivity difference into a memory and adding the data for correction of the sensitivity to, or subtracting the data from, the outputs from the sensor arrays. The sensitivity correction data stored into the memory corresponds to the output difference between the sensor arrays when uniform measuring light of given brightness is made to hit the sensor arrays, i.e., fixed values are adopted.
With the aforementioned sensitivity difference correction technique (i.e., sensitivity correction data about fixed values is added to, or subtracted from, the outputs from the sensor arrays), however, any appropriate correction cannot be made for normal sensor arrays producing outputs whose difference increases with increasing the incident light amount. For example, where the technique is applied to a pair of sensor arrays whose outputs are both zero when no incident light amount is present and the difference between the outputs increases with increasing the amount of incident light, the problem that sensitivity correction data about fixed values in the absence of incident light amount appear directly as output values takes place. This makes it impossible to make an appropriate correction. Therefore, where the aforementioned sensitivity difference correction technique is applied to a rangefinder using these sensor arrays, a subject of given brightness can be corrected for sensitivity at high accuracy when sensitivity correction data is created. However, subjects of other brightness cannot be corrected for sensitivity at high accuracy. That is, the sensitivity cannot be corrected according to the amount of incident light. Accordingly, where the sensitivity difference correction technique mentioned above is used, the rangefinding accuracy will vary according to the incident light amount. Also, in a focusing device for focusing the objective lens based on the output of a rangefinder, improvement of the focusing accuracy will not be expected.